Partial purification and characterization of L-histidine decarboxylase from fetal rats

Histamine synthesis in intact and disrupted rat mast cells

Histamine production by purified intact rat peritoneal mast cells, as measured by formation of [beta-3H]histamine from [beta-3H]L-histidine or by release of 14CO2 from 14C-carboxyl-labeled histidine, was ten to thirty times greater than that of disrupted cells of soluble extracts of these cells. Loss of activity was evident whether cells were disrupted by sonification, freezing and thawing, or lysis, both in the absence and presence of inhibitors of proteolytic enzymes and agents known to preserve enzyme responsible for histamine formation in both the intact cells and cell extracts. In the presence of subsaturating concentrations of histidine, various histidine analogs and glutamine inhibited histidine data indicate that, at physiological concentrations of histidine, blockade of histidine transport (through system N) may limit histamine synthesis in the intact cell and that measurement of histidine decarboxylase activity in tissue homogenates or cell extracts may not reflect actual histidine decarboxylase activity in vivo.

Properties of histidine decarboxylase from rat gastric mucosa

The properties of specific histidine decarboxylase from highly purified rat gastric mucosa preparations were studied. The kinetic parameters were pH dependent: the apparent Km value varied inversely with pH; the maximum reaction velocity was reached at pH 6.6; the optimum pH was related to substrate concentration. The enzyme was unstable below pH 5.5. The effect of temperature was investigated and the enzyme activity was optimum near 56 degrees C. The thermal inactivation of the enzyme showed the presence of several active forms displaying distinct thermostabilities. The effect of coenzyme and substrate on heat stability was established. A small amount of pyridoxal phosphate was required for maximum enzyme activity, and the Km was low. The cofactor appeared to be tightly bound to the apoenzyme; nevertheless there was a fraction more easily resolved by dialysis. With high pyridoxal phosphate concentrations non-competitive inhibition occurred. Histamine inhibited the enzyme at high concentrations, the inhibition being competitive with respect to the substrate. No metal ion was required for enzyme activity; the enzyme was inhibited by sulfhydryl reagents and heavy metal ions, and also by high concentrations of reducing agents. The tryptophan residue of the holoenzyme seemed to be essential for the catalytic process.

Absence of increase of histidine decarboxylase activity in mast cell-deficient W/W mouse embryos before parturition

The histidine decarboxylase (L-histidine carboxylase, EC 4.1.1.22) activity of whole W/W mouse embryos, which are devoid of mast cells, remained very low and did not show the rapid increase before birth (17-20 days of gestation) seen in wild-type +/+ embryos. During the same period, the histamine content also remained very low and no mast cells were detected in the W/W embryos, in the contrast to the large increase in both histamine content and number of mast cells in wild-type embryos. These findings imply that the histamine in embryos is largely derived from mast cells. In +/+ mice, histidine decarboxylase activity decreased rapidly soon after birth without concomitant decrease in histamine content or number of mast cells, suggesting that the enzyme activity in mast cells is regulated by some unknown mechanism.